"I think it's worth remembering that the hijackers who struck the World Trade Center came here legally," Clinton told the crowd at a Boulder, Colo., campaign rally Tuesday. "We have to be attune to threats inside and outside and how long before any refugees will be vetted to come to the United States."

Since the terrorist attacks in Paris two weeks ago, Clinton has become vocal about her support for refugees entering the United States, as GOP presidential candidates and over half the nation's governors have expressed concern about their admission. During the second Democratic primary debate, Clinton called for the administration to increase the number of refugees let into the United States from 10,000 to 65,000.